1. Field of the Invention
The present disclosure relates to an inkjet head unit and an inkjet apparatus. Especially, the inkjet head unit can be replaced with a new one as a unit to the inkjet apparatus.
2. Description of the Related Art
Inkjet apparatuses each having an inkjet head for discharging ink on a medium are known. The medium is a material onto which the discharged ink impacts. The inkjet apparatuses have been increasingly used in many industries. In the present invention, the media includes all media used in those industries as the materials onto which ink is to be discharged. To facilitate understanding, in the descriptions below, a recording medium such as recording paper is described as an example.
An inkjet apparatus described as an example has a carriage that reciprocates over a recording medium. On the carriage, an inkjet head is mounted. The inkjet head has a discharge pressure generation element for generating a pressure for discharging ink, and a discharge nozzle. The discharge pressure generation element operates in conjunction with the reciprocating operation of the carriage, and the discharge nozzle discharges ink droplets.
The recording medium is intermittently conveyed in conjunction with the carriage operation. The reciprocating operation of the carriage and the conveyance of the recording medium are alternately performed to form characters and images on the recording medium.
The inkjet apparatus includes an ink supply container (hereinafter, referred to as an ink tank). The ink tank can be expressed as an ink retaining unit in terms of a function of retaining the ink. The attached ink tank is connected to the inkjet head via a liquid conducting tube formed in a hollow shape using a flexible material. The ink retained in the ink tank passes through the liquid conducting tube, and is supplied to the inkjet head.
In such an inkjet apparatus, the amount of the ink droplets and the discharge direction can be stabilized by forming a meniscus with a good ink condition in the discharge nozzle at the opening of the discharge nozzle of the inkjet head. The stable ink discharge stabilizes the quality of the image formed on the recording medium. The formation of the meniscus with the good ink condition in the discharge nozzle can be implemented, for example, by maintaining the inside of the inkjet head in a slightly negative pressure state.
When the discharge of the ink is continued, the condition of the ink can be changed to the following conditions. For example, in the discharge nozzle, the solvent in the ink vapors in the air and thereby the viscosity of the ink increases, bubbles accumulate in an ink chamber near the discharge pressure generation element or in an ink chamber to which the discharge pressure generation element is provided, or dusts exist near the ink chamber or the discharge nozzle.
In such conditions, it is known that the inkjet head causes decrease in the quality of a recorded image. In other words, under such conditions, the direction of the ink droplets discharged from the discharge nozzle can deviate from the expected discharge direction.
Due to the deviation of the discharge direction, the impact positions of the ink droplets on the recording medium deviate from expected positions. As a result, the quality of the recorded image decreases. If the above conditions of the discharge nozzle proceed and become worse, the ink droplets may not be discharged from the discharge nozzle. In the specification, for convenience, such a condition is expressed as clogging of the discharge nozzle.
The Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-507522 discusses a structure for circulating ink between an ink tank and an inkjet head while the inside of the inkjet head is maintained in a slightly negative pressure state. The circulation of the ink provides an environment in which the above-mentioned undesirable conditions in the discharge nozzle can be reduced, and thereby the decrease in the quality of the recorded image can be reduced.
An inkjet apparatus discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-507522 includes a pressure pump for applying pressure to ink in an ink tank and feeding the ink to an inkjet head, and a pressure adjustment unit provided in the middle of the channel from the inkjet head to the ink tank.
The pressure adjustment unit maintains the inside of the inkjet head in a slightly negative pressure state using a water head difference between the ink liquid level of a discharge nozzle in the inkjet head and the ink liquid level in the pressure adjustment unit. In other words, the inkjet apparatus performs the ink circulation and the ink pressure adjustment using the pressure pump and the water head difference type pressure adjustment unit.
However, in the water head difference type pressure adjustment unit, the pressure in the inkjet head cannot be maintained in a predetermined slightly negative pressure state if the ink tank is not disposed at a position lower than the discharge surface (the plane the discharge nozzle is formed) of the inkjet head in the direction of gravitational force. Consequently, in the inkjet apparatus discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-507522, the installation location of the pressure adjustment unit is limited to a position lower than the inkjet head in the direction of gravitational force.
Further, in the head difference type pressure adjustment unit, the pressure in the inkjet head is determined depending on the distance between the discharge surface of the inkjet head and the pressure adjustment unit in the direction of gravitational force. Consequently, if the pressure in the inkjet head is to be maintained at a low negative pressure, the pressure adjustment unit is to be disposed at a further lower position in the direction of gravitational force. As a result, due to the limitation in the required size of the inkjet apparatus, and the like, the installation location of the pressure adjustment unit is further limited.
To solve the problems, the inventors tried to reduce the pressure of the ink in the inkjet head using a decompression pump.
However, when the decompressing pump is directly connected to the inkjet head, there is a possibility that the pressure of the ink in the inkjet head would decrease too low depending on the output of the decompression pump. If the pressure decreases too low, the meniscus formed at the discharge nozzle is pulled too much, and this makes the amount of the ink droplets to be discharged from the inkjet head unstable. As a result, the quality of the recorded image decreases.
Especially, a good meniscus in the discharge nozzle is formed at a slightly negative pressure less than atmospheric pressure by about 0.6 kPa. Unfortunately, at present, it is very difficult to stably adjust the pressure to the slightly negative pressure by controlling the output of the decompression pump.